This application is based on Japanese Patent Application No. 10-8383 filed Jan. 20, 1998, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates in general to a braking system for an automotive vehicle, which is equipped with a vacuum booster, and more particularly to a vehicle braking system capable of preventing a reduction in braking effect due to reduction in the boosting function of the vacuum booster.
2. Discussion of the Related Art
One example of a known braking system of the type indicated above is disclosed in JP-A-9-30385. This braking system includes (a) a brake operating member, (b) a master cylinder, (c) a vacuum booster for boosting a brake operating force acting on the brake operating member, on the basis of a pressure difference between a pressure in a negative-pressure chamber communicating with a negative-pressure source, and a pressure in a variable-pressure chamber selectively communicating with the negative-pressure chamber and an atmosphere, so that the boosted force is applied to the master cylinder, (d) a wheel brake cylinder connected to the master cylinder through a fluid passage and actuated by a hydraulic pressure applied thereto through the fluid passage, for braking a wheel of an automotive vehicle, and (e) a pressure increasing device actuated when a predetermined condition is satisfied during an operation of the brake operating member, to initiate a pressure increasing operation of increasing a hydraulic pressure in the wheel brake cylinder, with a hydraulic pressure source other than the master cylinder, such that the increased wheel brake cylinder pressure is higher than a hydraulic pressure generated by the master cylinder. The brake operating force or other quantity relating to this force, at which the predetermined condition is satisfied, is referred to as xe2x80x9cpressure increase initiating pointxe2x80x9d.
The known braking system indicated above is adapted such that the brake operating force acting on the brake operating member is boosted by the vacuum booster at a predetermined boosting ratio, until a boosting limit of the vacuum booster is reached, and such that the pressure increasing device initiates the pressure increasing operation when the boosting limit is reached, namely, when the brake operating force or other quantity has increased to the pressure increase initiating point, which is reached when the boosting limit is reached.
In this known braking system, a fact that the boosting limit of the vacuum booster is reached is detected when one of the following three conditions is satisfied: (1) the condition that a pressure switch has detected that the pressure in the variable-pressure chamber has been raised to the atmospheric pressure; (2) the condition that a switch has detected that an input rod of the vacuum booster which is operatively connected to the brake operating member has been brought into abutting contact with an output rod of the vacuum booster which is operatively connected to the master cylinder; and (3) the condition that the actual wheel brake cylinder pressure detected by a pressure sensor has become lower than a nominal value corresponding to the brake operating force detected by a pressure sensor. In the braking system described above wherein the moment at which the boosting limit of the vacuum booster is reached changes with a change in the pressure in the negative-pressure chamber, the pressure increase initiating point changes depending upon the pressure in the negative-pressure chamber. Theoretically, therefore, a change in the pressure in the negative-pressure chamber does not have influences on the braking effect provided by the wheel brake cylinder and on the operating feel of the brake operating member as given to the vehicle operator.
Actually, however, it is difficult to accurately detect that the boosting limit of the vacuum booster has been reached. Accordingly, the known braking system inevitably suffers from a problem that the braking effect and the brake operating feel are influenced by a change in the negative-pressure chamber of the vacuum booster.
It is therefore an object of this invention to provide a braking system adapted to reduce or prevent influences of a change in the pressure in the negative-pressure chamber of the vacuum booster, on the braking effect and the brake operating feel.
The above object may be achieved according to any one of the following modes of the present invention, each of which is numbered like the appended claims and refers to or depends from the other mode or modes, where appropriate, so as to indicate possible combinations of elements or features in preferred forms of the invention.
(1) A braking system for braking a wheel of an automotive vehicle, comprising:
a brake operating member;
a master cylinder functioning as a first hydraulic pressure source for generating a hydraulic pressure based on an input force;
a vacuum booster having a negative-pressure chamber connected to a negative pressure source, and a variable-pressure chamber selectively connected to the negative-pressure chamber and an atmosphere, the vacuum booster boosting an operating force of the brake operating member on the basis of a difference between pressures in the negative-pressure and variable-pressure chambers, and transmitted the boosted operating force to the master cylinder, the vacuum booster having a transition point at which a boosting ratio of said vacuum booster is reduced during an increase of the brake operating force before a boosting limit point at which the booster has become inoperable to perform its boosting function is reached, the transition point being kept unchanged irrespective of a change in the pressure in the negative-pressure chamber;
a brake including a wheel brake cylinder which is connected to the master cylinder through a fluid passage and which is activated by a hydraulic pressure supplied thereto through the fluid passage, for braking the wheel; and
a pressure increasing device including a second hydraulic pressure source connected to the fluid passage, the pressure increasing device performing a pressure increasing operation of increasing a hydraulic pressure in the wheel brake cylinder, by using the second hydraulic pressure source, such that the increased hydraulic pressure in the wheel brake cylinder is higher than that generated by the master cylinder, the pressure increasing device initiating the pressure increasing operation when the brake operating force has increased to the transition point.
In the braking system of the present invention constructed as described above, the transition point at which the boosting ratio of the vacuum booster is reduced during an increase of the brake operating force does not change with a change in the pressure in the negative-pressure chamber, while the boosting limit point changes with the pressure in the negative-pressure chamber. When the brake operating force has increased to the transition point, the pressure increasing device initiates its pressure increasing operation. Accordingly, the moment at which the pressure increasing operation of the pressure increasing device is initiated is stabilized. That is, the pressure increase initiating point of the pressure increasing device is stabilized, assuring increased stability of the braking effect and operating feel of the brake operating member.
(2) A braking system according to the above mode (1), wherein the vacuum booster is constructed such that an output of the vacuum booster corresponding to the transition point is smaller than that corresponding to the boosting limit point when the pressure in the negative-pressure chamber is equal to a lower limit of an expected range of variation of the pressure in the negative-pressure chamber.
In the braking system according to the above mode (2) of the invention, the vacuum booster is so constructed that the transition point of the vacuum booster is necessarily reached before the boosting limit point is reached, even if the pressure in the negative-pressure chamber is lowered to the expected lower limit. Accordingly, the pressure increasing device necessarily initiates its pressure increasing operation when the brake operating force has increased to the fixed transition point, irrespective of a variation in the pressure in the negative-pressure chamber.
(3) A braking system according to the above mode (1) or (2), which is arranged so as to brake the automotive vehicle at a deceleration value not lower than a maximum deceleration value during a normal operation of the brake operating member, if the boosting limit point of the vacuum booster is reached during the pressure increasing operation when the pressure in the negative-pressure chamber is equal to a lower limit of an expected range of variation of the pressure in the negative-pressure chamber.
In the braking system according to the above mode (3), the boosting limit point of the vacuum booster is not reached during a normal operation of the brake operating member even if the pressure in the negative-pressure chamber is lowered to the expected lowest value. Accordingly, during the normal braking operation, the present braking system does not suffer from a change in the braking effect and the brake operating feel, which would take place if the boosting limit point of the vacuum booster were reached.
The normal operation of the brake operating member is interpreted to mean an operation of the brake operating member while the vehicle is running on a normal or ordinary road surface.
The maximum deceleration value of the automotive vehicle is generally considered to fall within a range of 0.8 G-1.2 G, typically, about 1.0 G.
(4) A braking system according to any one of the above modes (1)-(3), wherein the pressure increasing device includes (a) an operating-force-related-quantity sensor for detecting an operating-force-related quantity relating to the brake operating force, and (b) pressure increase initiating means for commanding the pressure increasing device to initiate the pressure increasing operation when the operating-force-related quantity detected by the operating-force-related-quantity sensor has increased to a value corresponding to the transition point of said vacuum booster.
The operating-force-related-quantity sensor may be a sensor for detecting the brake operating force per se, a sensor for detecting an operating stroke of the brake operating member, a sensor for detecting the pressure in the master cylinder, or a sensor for detecting the deceleration value of the automotive vehicle.
(5) A braking system according to any one of the above modes (1)-(4), wherein the vacuum booster has a first boosting ratio at which the operating force of the brake operating member is boosted until the operating force has increased to the transition point, and a second boosting ratio which is lower than the first boosting ratio and at which the operating force is boosted while the operating force increases from the transition point up to the boosting limit point.
(6) A braking system according to the above mode (5), wherein the vacuum booster includes:
a housing;
an input rod operatively connected to the brake operating member;
a power piston axially movably disposed within the housing and cooperating with the housing to device the negative-pressure chamber and the variable-pressure chamber, the power piston being axially moved by the difference between the pressures in the negative-pressure and variable-pressure chambers;
a valve mechanism for selectively connecting the variable-pressure chamber to the negative-pressure chamber and the atmosphere, on the basis of a relative axial movement of the power piston and the input rod;
an output rod for transmitting an operating force of the power piston to a pressurizing piston of the master cylinder; and
an elastic reaction disc disposed such that the power piston and the input rod contact the reaction disc in one of opposite axial directions of the vacuum booster while the output rod contacts the reaction disc in the other axial direction,
and wherein one of opposite end portions of the input rod at which the input rod contacts the reaction disc consists of two sections one of which is located adjacent to the reaction disc and has a first transverse cross sectional area, and the other of which is remote from the reaction disc and has a second transverse cross sectional area larger than the first transverse cross sectional area.
(7) A braking system according to any one of the above modes (1)-(4), wherein the boosting ratio of the vacuum booster is continuously reduced as the operating force of the brake operating member increases, and the transition point is reached when the boosting ratio has been reduced to a value which is not zero.
(8) A braking system according to the above mode (7), wherein the vacuum booster includes:
a housing;
an input rod operatively connected to the brake operating member;
a power piston axially movably disposed within the housing and cooperating with the housing to device the negative-pressure chamber and the variable-pressure chamber, the power piston being axially moved by the difference between the pressures in the negative-pressure and variable-pressure chambers;
a valve mechanism for selectively connecting the variable-pressure chamber to the negative-pressure chamber and the atmosphere, on the basis of a relative axial movement of the power piston and the input rod;
an output rod for transmitting an operating force of the power piston to a pressurizing piston of the master cylinder; and
an elastic reaction disc disposed such that the power piston and the input rod contact the reaction disc in one of opposite axial directions of the vacuum booster while the output rod contacts the reaction disc in the other axial direction,
and wherein one of opposite end portions of the input rod at which the input rod contacts the reaction disc has a transverse cross sectional area which continuously increases in an axial direction of the input rod from the one of the opposite end portions toward the other end portion.
(9) A braking system according to any one of the above modes (1)-(8), wherein the pressure increasing device is a pump type pressure increasing device including a fluid flow control valve provided in the fluid passage, and a pump functioning as the second hydraulic pressure source and having a delivery end connected to a portion of the fluid passage between the fluid flow control valve and the wheel brake cylinder, the pump being operated to increase the pressure in the wheel brake cylinder to a level higher than the pressure in the master cylinder, while the fluid flow control valve inhibits a flow of a working fluid at least in a direction from the wheel brake cylinder toward the master cylinder.
(10) A braking system according to the above mode (9), further comprising a flow restrictor which by-passes the fluid flow control valve.
In the braking system according to the above mode (10), the flow restrictor permits application of a hydraulic pressure corresponding to the pressure of the wheel brake cylinder to the master cylinder, so that the vehicle operator can feel an increase of the wheel brake cylinder pressure by operation of the pump.
The braking system may have the feature of the above mode (10), without the feature according to the above mode (1) that the pressure increasing operation is initiated when the brake operating force has increased to the transition point of the vacuum booster.
(11) A braking system according to the above mode (10), wherein the fluid flow control valve includes a solenoid-operated two-position valve having an open position and a closed position.
(12) A braking system according to the above mode (11), wherein the pump type pressure increasing device further includes two-position valve control means for normally holding the two-position valve in the open position, switching the two-position valve from the open position to the closed position when the pressure increasing operation is initiated by the pump type pressure increasing device, and switching the two-position valve from the closed position to the open position when the pressure increasing operation is terminated.
(13) A braking system according to the above mode (10), wherein the fluid flow control valve includes a solenoid-operated two-position valve having an open position and a closed position, and the pump type pressure including device includes (a) two-position valve control means for normally holding the two-position valve in the open position, switching the two-position valve from the open position to the closed position when the pressure increasing operation is initiated by the pump type pressure increasing device, and switching the two-position valve from the closed position to the open position when the pressure increasing operation is terminated, and (b) pump delivery control means for controlling a delivery amount of the pump to control a rate of flow of the working fluid through the flow restrictor, for thereby variably controlling a difference between the pressures in the master cylinder and the wheel brake cylinder.
In the braking system according to the above mode (13), the solenoid-operated two-position valve is used as the fluid flow control valve, but the pressure increasing operation can be achieved with a high degree of control freedom, by changing the difference between the pressure of the master cylinder and the pressure of the wheel brake cylinder, by effectively utilizing the flow restrictor which by-passes the two-position valve.
(14) A braking system according to any one of the above modes (1)-(13), wherein the pressure increasing device is a pump type pressure increasing device comprising a pressure control valve which is provided in the fluid passage and which includes (a) a valve member and a valve seat which are spaced apart from each other, so as to permit a flow of a working fluid from the wheel brake cylinder toward the master cylinder, when a pressure difference which is the pressure in the wheel brake cylinder minus the pressure in the master cylinder is larger than a predetermined threshold, the valve member and the valve seat fluid-tightly contacting each other, so as to inhibit the flow of the working fluid, when the difference is not larger than the predetermined threshold, and (b) a pressure difference controlling device for generating a magnetic force for biasing the valve member and the valve seat toward each other, and continuously controlling the magnetic force to continuously change the pressure difference which is established when the valve member and the valve seat initiates a fluid-tight contact therebetween,
and wherein the pump type pressure increasing device further comprises a pump functioning as the second hydraulic pressure source and having a delivery end connected to a portion of the fluid passage between the pressure control device and the wheel brake cylinder.
In the braking system according to the above mode (14), the pressure difference of the master cylinder and the wheel brake cylinder can be continuously changed, so that the pressure of the wheel brake cylinder can be increased with improved accuracy.
Further, the pressure of the wheel brake cylinder can be changed following the master cylinder pressure, by a mechanical operation of the pressure control device. Accordingly, the present arrangement wherein the wheel brake cylinder pressure mechanically follows the master cylinder pressure permits reduced complexity of electric control of the pressure difference controlling device, whereby the accuracy of control of the wheel brake cylinder pressure by the pressure increasing device can be further improved.
(15) A braking system according to the above mode (14), wherein the pressure control valve further includes a disabling device for preventing the valve member and the valve seat from fluid-tightly contacting each other to thereby disable the pressure control device, when the pump type pressure increasing device is not required to be operated to perform the pressure increasing operation.
In the braking system according to the above mode (15), the disabling device mechanically disables the pressure control valve when the pressure increasing operation is not necessary, so that the pressure control valve does not disturb an operation of the braking system when the pressure increasing operation of the pressure increasing device is not necessary.
(16) A braking system according to the above mode (15), wherein the pressure difference controlling device includes a solenoid coil, and the disabling device includes an elastic member for biasing the valve member and the valve seat away from each other.
(17) A braking system according to any one of the above modes (1)-(16), wherein the pressure increasing device comprises (a) electrically operated pressure control device for controlling the pressure in the wheel brake cylinder, and (b) a pseudo pressure holding means operated when the pressure in the wheel brake cylinder is required to be held at a presently established level, for electrically controlling the electrically operated pressure control device, so as to hold the pressure in the wheel brake cylinder, without using a pressure holding valve which disconnects the wheel brake cylinder from the master cylinder and the electrically operated pressure control device.
The braking system according to the above mode (17) does not necessarily require an exclusive pressure holding valve for holding the pressure of the wheel brake cylinder, since the electrically operated pressure control device for controlling the pressure of the wheel brake cylinder can be controlled by the pseudo pressure holding means so as to hold the wheel brake cylinder pressure. Accordingly, the braking system can be simplified in construction and is available at an accordingly reduced cost.
The braking system may have the feature of the above mode (17), without the feature according to the above mode (1) that the pressure increasing operation is initiated when the brake operating force has increased to the transition point of the vacuum booster. Further, the feature of the present mode (17) may be effective not only in increasing the wheel brake cylinder pressure so as to be higher than the master cylinder pressure, for thereby compensating for reduction of the boosting function of the vacuum booster, but also for any other purpose in controlling the wheel brake cylinder pressure. For example, this feature may be effective in controlling the wheel brake cylinder pressure in an anti-lock control fashion so as to prevent an excessive locking tendency of the vehicle wheel.
(18) A braking system according to the above mode (17), wherein the pressure increasing device is a pump type pressure increasing device including a fluid flow control valve provided in the fluid passage, and a pump functioning as the second hydraulic pressure source and having a delivery end connected to a portion of the fluid passage between the fluid flow control valve and the wheel brake cylinder, the pump being operated to increase the pressure in the wheel brake cylinder to a level higher than the pressure in the master cylinder, while the fluid flow control valve inhibits a flow of a working fluid at least in a direction from the wheel brake cylinder toward the master cylinder, and wherein the electrically operated pressure control device includes the fluid flow control valve and the pump, and the pseudo pressure holding means controls at least one of the fluid flow control valve and the pump, to hold the pressure in the wheel brake cylinder.
In the braking system according to the above mode (18), the pressure in the wheel brake cylinder can be held at the present level, by utilizing at least one of the fluid flow control valve and the pump which are provided for increasing the wheel brake cylinder pressure. Accordingly, the present braking system does not require an exclusive pressure holding valve, making it possible to simplify the construction of the braking system and reduce the cost of manufacture.
(19) A braking system according to the above mode (18), wherein the pseudo pressure holding means holds the pressure in the wheel brake cylinder, by controlling the fluid flow control valve so as to disconnect the wheel brake cylinder from the master cylinder, and by holding the pump in a non-operated state.
(20) A braking system according to the above mode (18), wherein the pseudo pressure holding means holds the pressure in the wheel brake cylinder, by holding the pump in an operated state and controlling the fluid flow control valve such that the working fluid delivered from the pump leaks into the master cylinder through the fluid flow control valve.
(21) A braking system according to any one of the above modes (17)-(20), wherein the pressure increasing device is a pump type pressure increasing device including a fluid flow control valve provided in the fluid passage, and a pump functioning as the second hydraulic pressure source and having a delivery end connected to a portion of the fluid passage between the fluid flow control valve and the wheel brake cylinder, the pump being operated to increase the pressure in the wheel brake cylinder to a level higher than the pressure in the master cylinder, while the fluid flow control valve inhibits a flow of a working fluid at least in a direction from the wheel brake cylinder toward the master cylinder, the braking system further comprising:
a reservoir connected to the wheel brake cylinder;
a pressure reducing valve disposed between the reservoir and the wheel brake cylinder, and having a closed state inhibiting a flow of the working fluid from the wheel brake cylinder toward the reservoir, and an open state for permitting the flow of the working fluid,
and wherein the electrically operated pressure control device includes the pressure reducing valve, the fluid flow control valve and the pump, and the pseudo pressure holding means holds the pressure in the wheel brake cylinder, by electrically controlling at least one of the pressure reducing valve, the fluid flow control valve and the pump.
(22) A braking system according to the above mode (21), wherein the pseudo pressure holding means includes means for controlling the fluid flow control valve so as to disconnect the wheel brake cylinder from the master cylinder, for holding the pump in an operated state, and for opening the pressure reducing valve.
(23) A braking system according to any one of the above modes (17)-(22), wherein the pressure increasing device is a pump type pressure increasing device including a fluid flow control valve provided in the fluid passage, and a pump functioning as the second hydraulic pressure source and having a delivery end connected to a portion of the fluid passage between the fluid flow control valve and the wheel brake cylinder, the pump being operated to increase the pressure in the wheel brake cylinder to a level higher than the pressure in the master cylinder, while the fluid flow control valve inhibits a flow of a working fluid at least in a direction from the wheel brake cylinder toward the master cylinder, the braking system further comprising:
an inflow control valve connected to a suction side of the pump, and having a closed state for inhibiting a flow of the working fluid toward the suction side of the pump, and an open state for permitting the flow of the working fluid toward the suction side to thereby permit the pump to deliver the working fluid,
and wherein the electrically operated pressure control device includes the inflow control valve, the fluid flow control valve and the pump, and the pseudo pressure holding means holds the pressure in the wheel brake cylinder, by electrically controlling at least one of the inflow control valve, the fluid flow control valve and the pump.
(24) A braking system according to the above mode (23), wherein the pseudo pressure holding means includes means for controlling the fluid flow control valve so as to disconnect the wheel brake cylinder from the master cylinder, for holding the pump in an operated state thereof, and for closing the inflow control valve.
(25) A braking system according to any one of the above modes (17)-(24), further comprising an anti-lock braking pressure control device for controlling the pressure in the wheel brake cylinder so as to prevent an excessive locking tendency of the wheel during an operation of the brake operating member, and wherein the anti-lock braking pressure control device commands the pseudo pressure holding means for holding the pressure in the wheel brake cylinder, during an operation of the anti-lock braking pressure control device.
(26) A braking system according to any one of the above modes (1)-(25), wherein the pressure increasing device is a pump type pressure increasing device including a fluid flow control valve provided in the fluid passage, and a pump functioning as the second hydraulic pressure source and having a delivery end connected to a portion of the fluid passage between the fluid flow control valve and the wheel brake cylinder, the pump being operated to increase the pressure in the wheel brake cylinder to a level higher than the pressure in the master cylinder, while the fluid flow control valve inhibits a flow of a working fluid at least in a direction from the wheel brake cylinder toward the master cylinder,
and wherein a plurality of wheel brake cylinders are connected through the fluid passage to a pressurizing chamber of the master cylinder, and the fluid flow control valve and the pump are provided for each of the plurality of wheel brake cylinders, such that the fluid flow control valve and the pump for each one of the plurality of wheel brake cylinders are controllable independently of the fluid flow control valve and the pump for another of the plurality of wheel brake cylinders.
In the braking system according to the above mode (26), the operations to increase the pressures in the wheel brake cylinders connected to the same pressurizing chamber of the master cylinder are controlled independently of each other, by the corresponding set of fluid flow control valve and pump, such that the pressure in each of those wheel brake cylinders is not influenced by the pressure in the other wheel brake cylinder or cylinders, unlike the wheel brake cylinder pressures in the case where these wheel brake cylinders pressures are all controlled by only one set of fluid flow control valve and pump. Accordingly, the accuracy of control to increase the pressure in each wheel brake cylinder is improved.
The braking system may have the feature of the above mode (26), without the feature according to the above mode (1) that the pressure increasing operation is initiated when the brake operating force has increased to the transition point of the vacuum booster. Further, the feature of the present mode (26) may be effective not only for increasing the wheel brake cylinder pressure so as to be higher than the master cylinder pressure, for thereby compensating for reduction of the boosting effect of the vacuum booster, but also for any other purpose in controlling the wheel brake cylinder pressure.
(27) A braking system according to any one of the above modes (1)-(26), wherein the pressure increasing device is a pump type pressure increasing device including a fluid flow control valve provided in the fluid passage, and a pump functioning as the second hydraulic pressure source and having a delivery end connected to a portion of the fluid passage between the fluid flow control valve and the wheel brake cylinder, the pump being operated to increase the pressure in the wheel brake cylinder to a level higher than the pressure in the master cylinder, while the fluid flow control valve inhibits a flow of a working fluid at least in a direction from the wheel brake cylinder toward the master cylinder,
and wherein the automotive vehicle has a front wheel and a rear wheel, and a front wheel brake cylinder and a rear wheel brake cylinder are provided for braking the front and rear wheels, respectively, the front wheel brake cylinder having a larger volume than the rear wheel brake cylinder, the fluid flow control valve and the pump are provided for each of the front and rear wheel brake cylinders, such that the fluid flow control valve and the pump for each of the front and rear wheel brake cylinders are controllable independently of the fluid flow control valve and the pump for the other of the front and rear wheel brake cylinders, the pump type pressure increasing device including pressure increasing rate control means for controlling a rate of increase of the pressure in the each of the front and rear wheel brake cylinders such that the rates of increase of the pressures in the front and rear wheel brake cylinders are equal to each other, irrespective of a difference between the volumes of the front and rear wheel brake cylinders.
In the braking system according to the above mode (27), the fluid flow control valve and the pump are provided for each of the front and rear wheel brake cylinders, such that the two sets of the fluid flow control valve and pump for the respective front and rear wheel brake cylinders are controllable independently of each other. Further, the rates of increase of the pressures in the front and rear wheel brake cylinders are controlled by the pressure increasing rate control means such that these rates are equal to each other, irrespective of the volume difference of the front and rear wheel brake cylinders.
The braking system may have the feature of the above mode (27), without the feature according to the above mode (1) that the transition point of the vacuum booster is kept unchanged irrespective of a change in the pressure in the negative-pressure chamber. Further, the feature of the present mode (27) may be effective not only for increasing the wheel brake cylinder pressure so as to be higher than the master cylinder pressure, after reduction of the boosting ratio of the vacuum booster, but also for any other purpose in controlling the wheel brake cylinder pressure.
(28) A braking system according to the above mode (27), wherein a plurality of solenoid-operated flow control valves are connected to suction sides of the pumps for the front and rear wheel brake cylinders, independently of each other, and the pressure increasing rate control means electrically controls the plurality of solenoid-operated flow control valves such that a rate of flow of the working fluid into the pump for the front wheel brake cylinder is higher than a rate of flow of the working fluid into the pump for the rear wheel brake cylinder.
(29) A braking system according to the above mode (27), wherein the pressure increasing rate control means electrically controls the pumps for the respective front and rear wheel brake cylinders such that a delivery amount of the pump for the front wheel brake cylinder is larger than a delivery amount of the pump for the rear wheel brake cylinder.
(30) A braking system according to the above mode (27), wherein the pressure increasing rate control means electrically controls the fluid flow control valves for the front and rear wheel brake cylinders, such that a rate of flow of the working fluid from the pump for the front wheel brake cylinder into the master cylinder through the fluid flow control valve for the front wheel brake cylinder is lower than a rate of flow of the working fluid from the pump for the rear wheel brake cylinder into the master cylinder through the fluid flow control valve for the rear wheel brake cylinder.
(31) A braking system according to the above mode (27), wherein the pressure increasing rate control means includes the pumps for the front and rear wheel brake cylinders, the pump for the front wheel brake cylinder having a larger delivery amount than the pump for the rear wheel brake cylinder when the pumps are held operated at a same speed.
According to the present invention, there are also provided the following braking systems:
(32) A braking system for braking a wheel of an automotive vehicle, comprising:
a brake operating member;
a master cylinder functioning as a first hydraulic pressure source for generating a hydraulic pressure based on an input force;
a brake including a wheel brake cylinder which is connected to the master cylinder through a fluid passage and which is activated by a hydraulic pressure supplied thereto through the fluid passage, for braking the wheel; and
a pressure increasing device including a second hydraulic pressure source connected to the fluid passage, the pressure increasing device increasing a hydraulic pressure in the wheel brake cylinder, by using the second hydraulic pressure source, such that the increased hydraulic pressure in the wheel brake cylinder is higher than that generated by the master cylinder,
and wherein the pressure increasing device is a pump type pressure increasing device including a fluid flow control valve provided in the fluid passage, and a pump functioning as the second hydraulic pressure source and having a delivery end connected to a portion of the fluid passage between the fluid flow control valve and the wheel brake cylinder, the pump being operated to increase the pressure in the wheel brake cylinder to a level higher than the pressure in the master cylinder, while the fluid flow control valve inhibits a flow of a working fluid at least in a direction from the wheel brake cylinder toward the master cylinder,
the pump type pressure increasing device further including (a) target pressure increase amount determining means for determining, on the basis of a braking effect desired by an operator of the automotive vehicle, a target pressure increase amount by which the pressure in the wheel brake cylinder is controlled to be higher than the pressure in the master cylinder, and (b) duty control means for controlling a duty ratio of the pump, on the basis of a rate of change of the braking effect desired by the operator.
(33) A braking system according to the above mode (32), wherein the braking effect desired by the operator of the automotive vehicle is represented by a quantity relating to the pressure in the master cylinder during an operation of the brake operating member.
The quantity relating to the pressure in the master cylinder may be the master cylinder pressure per se, or any other physical quantity which changes with the master cylinder pressure, such as an operating force or stroke of the brake operating member, and a deceleration value of the automotive vehicle.
(34) A braking system according to the above mode (32) or (33), wherein the rate of change of the braking effect desired by the operator is represented by a rate of change of a quantity relating to the pressure in the master cylinder during an operation of the brake operating member.
(35) A braking system according to any one of the above modes (32)-(34), wherein the duty ratio control means includes means for determining the duty ratio of the pump such that a delivery amount of the pump increases with an increase in the rate of change of the braking effect desired by the operator.